WCA July 2012

The compositions of laboratory prepared ingots are shown in Table 1 and it should be noted that the ratios in the as-cast compositions were somewhat higher than designed, namely 1.44 and 2.39 respectively in the B and High B alloys. Free boron may hence also be present in the B alloy. The ingots were hot rolled on a hand charged rolling mill with reheating done at 1,176°C and reduction carried out in three steps on two hot rolling mills. Initially the bars were reduced from 12.7 to 9.5cm round corner square (RCS) followed by air cooling to room temperature, reheating and rolling to 4.76cm. The material was then machined to remove oxides and cut in 6 – 7 blocks. Final reduction was carried out on a second hot rolling mill to a final size of 7.1mm. The material was ambient air cooled after hot rolling. The material was then saw-cut to 3.7m lengths, prior to drawing. Twenty-four sections were obtained for each alloy. Although Thermo Calc ® thermodynamic calculations predicted a potential for hot shortness in the High B steel, no breakage or significant surface defects were observed. As significant decarburisation was observed, 8 the material was centreless ground to 5.5mm diameter. The hot rolled rods were then assessed for carbon segregation and only those rods with a carbon content of 0.78 ± 0.01 wt pct were retained for further wire drawing. Wire drawing was carried out at the Bekaert Technology Centre and involved reduction to 2.5mm diameter in eight drawing steps. Patenting was then conducted in salt baths with reheating at 980ºC followed by 520ºC. ❍ ❍ Figure 1 : Light optical micrographs of hot rolled rods Base, B and High B steels. Samples taken transverse to the rolling direction, in the centre of the cross section, 4% Picral etch

❍ ❍ Figure 2 : Transmission electron micrograph of the hot-rolled and air cooled high B material

The patented wire was then further drawn to 1mm.

Tensile testing was conducted on an electro-mechanical tensile machine at a constant strain rate of 5.6 10 -4 /s, with a 5cm 50% extensometer. Two samples were tested for each condition. Uniform strains were determined as the engineering strain at the peak load used for UTS calculations, and total strains to failure were obtained from the extensometer output at final fracture. Microstructural characterisation was done by light optical microscopy on 4% Picral etched samples and by transmission electron microscopy (TEM) on a Philips CM120 instrument operating at 120kV. Thin foils were electropolished with a Fischione twin-jet polisher operating at 32V at room temperature, using a mixture of 95 pct acetic and 5 pct perchloric acid. Dilatometry was carried out on a Gleeble ® 1500 system. Samples were reheated to 950°C at a constant heating rate of 20°C/s and held isothermally for five minutes. The steel was then cooled in helium gas at programmed constant cooling rates of 50, 30, 25, 12.5, 10, 7.5, 5, 2.5 and 1°C/s, respectively. Consecutive tests were conducted on a single specimen per alloy. The dilation of the sample was monitored with temperature and time. All samples were observed to fail within the specified extensometer gauge length unless otherwise stated.

Base

B

High

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Wire & Cable ASIA – September/October 2007 July/August 2012